Madman hit the nail on the head. Always use a pipette or syringe for the most consistent results. Start at 5% and work up from there... some flavors don't even need that much (coffees and tobaccos).
Everyone has their own way of mixing but here's how I do it.
I mix my base liquid with no nic. Since I like a 70/30 mix I have some of that mixed up (with zero nic) in advance just for DYI. I use that to experiment with so I'm not wasting expensive unflavored nic juice. Once I get a flavor I like I'll replace my unflavored pg with nic pg in the final mix. I end up throwing out more experiments than I keep and it's just cheaper this way.

I haven't noticed adding nic in the final mix changing the flavor much. Maybe a bit more throat hit but that's it. One other nice thing about doing it this way... you'll know right away if it's your unflavored nic that's causing the "odd taste".
If I'm mixing more than one flavor I do it the same way but once I figure out where I like it I'll mix up some of the "flavor base" of all those flavors in the same proportions in a 10ml bottle. Then when I want to mix some up in the future I just add this to my nic base. Makes it easy. Plus I think it allows the flavors to "pre-steep" and meld together even before you add the nic base.
Keep at it, you're on the right track.